Estimates are difficult for developers and product owners

One thing that changed my way of thinking about estimates is reading that 86% of engineering projects, regardless of what kind of engineering (chemical, infrastructure, industrial, etc) go over budget (in time or money).

Missing estimates isn't unique to software, but it's common across all engineering fields.

Physical real-world projects include a buffer for this, called "contingencies" or "change orders" so that if a requirement changes or they discover something like previously unknown site geology that will require changes to the foundation they can absorb it. Based on a large history of similar projects their estimates are usually pretty good but occasionally they will run over.